Amorphous silicon is one of the most promising materials for large area solar cells for terestrial photovoltaic applications. Unfortunately these cells suffer from two serious problems: the efficiencies drop when laboratory processes are scaled up and the cells degrade after some exposure to sunlight. The exact causes of these two problems are still unknown. In this project some aspects of the latter problem were investigated. The photo-degradation was investigated by illuminating films of a-Si:H with simulated sunlight for different periods of time and then thermally annealing them. The change in the optical properties were investigated with the aid of optical transmission spectroscopy. The films were also characterized by Fourier Transform Infra-Red (FTIR) spectroscopy. The change in the electrical properties of the intrinsic films was determined as function of temperature and total photon flux. No change in the optical properties could be detected. The illumination had-no effect on the FTIR measurements. It seems as if the hydrogen is not involved in the microscopic processes leading to the Staebler-Wronski Effect (SWE). The effect of the photo-degradation manifests itself in a drop in the the dark conductivity and photoconductivity over the total temperature range that was investigated. The observed phenomena are explained in terms of photo-induced deep levels in the gap. The Fermi level shifts to the middle of the gap due to these defect states, causing a drop in the free carrier concentration and conductivity. The measurements of photoconductivity as function of photon energy show that these defect levels increase the absorption coefficient in the long wavelength region, but they also decrease the lifetime of the photo-generated carriers. The photo-induced defects were investigated with the CPM-technique. It was found that the light introduced defects deep in the band gap. The concentration of the defects increases with illumination, but saturates after about 24 hours of illumination. The defects could be annealed almost completely. The microscopic processes causing the photo-degradation of α Si:H solar cells were investigated by comparing the different theoretical models explaining the SWE with the results obtained during this project.